This is a phase II trial to collect data that can be used to initiate further studies to determine the efficacy of Mycophenolate mofetil to treat moderate-severe pediatric inflammatory bowel disease. MM has not been used for disorders other than prevention and treatment of organ transplant rejection. However, it offers clear advantages over standard therapies using 6-MP or azathioprine. Preliminary data are necessary to embark on larger and more comprehensive investigations including a controlled cross-over trial with 6-MP.